The invention relates to a method for determining the azimuth of a borehole that is being drilled in a subsurface earth formation.
The invention relates in particular to a method for determining and correcting the influence of the erroneous magnetic field caused by magnetization of a drill string on an azimuth measurement made by a magnetic sensor package included in the drill string.
During deephole drilling operations it is general practice to survey from time to time the course of the borehole by means of a sensor package which is included in the drill string near the lower end thereof. The sensor package generally comprises a set of magnetometers that measure the components of the local magnetic field in three orthogonal directions. These measurements together with the direction of the earth magnetic field vector, and the direction of the local gravity vector, provide a suitable reference to determine the course of the borehole.
When measuring the orientation of the sensor package relative to the earth magnetic field vector while the drill string is present in the borehole the erroneous magnetic field caused by drill string magnetization may cause a significant error in the measurement. To reduce the magnitude of this error as much as possible it is current practice to arrange the sensor package in a drill collar which is made of non-magnetic material. Moreover, this collar is usually arranged in a drill string section comprising a series of non-magnetic collars to reduce the impact of the steel components of the drilling assembly, such as the drill bit and the drill pipes above the collars, on the magnetic field at the location of the sensors. A problem encountered when using non-magnetic drill collars is that these collars may become magnetized during drilling and in particular, the presence of so-called magnetic spots in the collar near the sensor assembly may impair the accuracy of the azimuth measurement considerably.
U.S. Pat. No. 4,163,324 describes a method for partially eliminating the error in the azimuth measurement caused by the erroneous magnetic field at the location of the sensor package, which field mainly is the result of drill string magnetization. In the patented method it is assumed that at the location of the sensors the vector of the erroneous magnetic field is oriented along the borehole axis. Although the known correction method generally enhances the accuracy of the azimuth measurement it does not correct for cross-axial magnetic error field. Said cross-axial magnetic error fields can originate from the presence of magnetic spots or steel components in the drilling assembly.